In a daily life and business activity, corner missing and tearing and so on of a banknote generally occur during a using process, and people generally bond the damaged or torn banknote using an adhesive tape or using a thin paper together with glue or the like, so as to continue to use the banknote. It is difficult to detect a banknote of which a surface is bonded with a foreign body effectively using a conventional mechanical apparatus. The banknote bonded with the foreign body is likely to result in damaging of a counterfeit detector and a sorting and identifying apparatus in a bank. If the banknote bonded with the foreign body is detected manually, it results in a waste of manpower and time and low detection efficiency. The financial industry proposes a requirement of “a quality of the circulated RMB should be superior than 70% of a quality of the new issued banknote” under support from the country, which requires financial self-service apparatus to improve the capability and level for identifying a banknote.
The conventional methods for detecting a foreign body on a surface of a banknote is by using a mechanical thickness measurement device, an infrared detection device or an ultrasound detection device. The mechanical thickness measurement device is difficult to detect a thin foreign body on the surface of the banknote due to low measurement accuracy. The infrared detection device has narrow application fields, and can detect only foreign bodies on the surface of the banknote having different directions and different reflection characteristics, such as transparent adhesive tapes. The ultrasound detection device has a long measurement wavelength and low accuracy due to using ultrasound as a carrier.